The present invention relates generally to testing systems for digital transmission lines used in telecommunication systems and, more particularly, to a testing system for an element interconnected to digital telecommunications transmission lines that determines whether or not there is continuity in the downstream transmission line spans. Many digital telecommunications transmission facilities include a central office which may transmit data signals over transmission lines to customer premises and channel service units. Typically, the signals are sent over the transmission lines differentially on two conductors, known as the Tip-Ring Pair.
The Bell telephone system in the United States, for example, has a widely utilized Digital "D" multiplexing pulse code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 transmission system. One pair of cables is provided for each direction of transmission.
For clarification and simplification of terminology, the pair of cables carrying signals from the central office to the customer premises equipment is designated as a "transmit" line, and the pair of cables transmitting data from the customer premises equipment and channel service unit to the central office is designated as a "receive" line. This designation is made for convenience only; of course, when an observer changes ;position from a central office to the customer premises, what used to be a "transmit" line becomes a "receive" line, and the "receive" line becomes a "transmit" line.
A variety of transmission line elements are interconnected to the transmission cables between the central office and the customer premises. For example, the signals sent by the central office to the customer premises (and vice versa) must regularly be regenerated by units called repeaters. Such regenerative repeaters are spaced along the transmission lines every 5,000 to 6,000 feet. A span may therefore be considered to consist of two pairs of cables between two system elements, such as repeaters, or between a repeater and a network interface unit.
The first repeater receives the data from the central office, but, because of transmission line losses, jitter, noise, interference, and distortion, the signal will have degraded. The repeater recognizes the presence or absence of a pulse at a particular point in time and, if appropriate, regenerates a relatively clean, new pulse. The regenerative repeater (or "line repeater" or "repeater") is powered through the transmission cable itself.
The power to operate the repeaters is provided by the central office using the simplex method. Accordingly, in the central office, a substantially constant-current source feeds the lines, and the current is typically returned, or "looped," at the customer premises equipment. Thus, direct current flows from the central office to the customer premises down the "transmit" spans, loops back at or near the customer premises, and returns back to the central office via the "receive" lines.
Repeaters are typically configured for "through powering." In such a case, a load, such as, for example, a zener diode, is used in a repeater in a "series" position. The load is in series with (1) the transmit span between the central office and the repeater ("upstream transmit span") and (2) the transmit span between the repeater and the customer premises ("downstream transmit span"). Occasionally, however, a break occurs in a span between repeaters. This may be due, for example, to a construction crew cutting a cable while digging or to a tree limb falling across the transmission line cables and breaking them. When this occurs, the through powered line elements necessarily lose power.
The telephone company, of course, wishes to be able to determine the location of the cable break before dispatching a crew to find and repair the break. Some repeaters (or other circuit elements) allow telephone operating companies to locate cable breaks (or "segment the break" to a particular span between two circuit elements such as repeaters). In such a case, the repeaters fall back to "loop powering" when a cable break occurs in a downstream span of cable. Instead of being in series with the transmit spans, the repeater load is switched between the upstream transmit and receive spans. Accordingly, the circuit elements between the break and the central office remain powered.
The repeaters may then be interrogated by test equipment (in the central office or elsewhere), via digital maintenance codes, to determine which repeaters are still receiving power. If, for example, the fourth repeater from the central office responds to interrogations and indicates that it is still receiving power, but the fifth circuit element does not respond at all (indicating that it is not receiving power), a tester in the central office may then determine that a break in the transmission lines occurred between the fourth and fifth repeaters.
After the repair in the transmission lines has been repaired, however, each of the line elements that have moved to a loop powered mode of operation should eventually return to a through powered mode of operation. Thus, repeaters should be able to independently investigate whether or not the cable break has been repaired and, thus, whether or not they should return to a through powered mode from a loop power mode.
Many presently existing methods used by line elements to determine whether the downstream transmission lines have been repaired, however, often involve adding an additional load (such as a "super zener diode") to the load that the repeater already has between the receive and transmit spans. This may accordingly involve wasting power and generating heat.
In the field, up to twelve repeaters may be held in a single case. Further, repeaters are often outside, and thus, must be able to operate in environmental conditions reaching up to 60.degree. centigrade or more. If a cable is cut and all repeaters in the apparatus move into a loop powered mode of operation, rather than a through powered mode of operation, the added loads may result in a significant temperature rise within the repeater, especially where many of the repeaters are physically grouped together and where the repeaters are positioned outside during the summer. In such a case, the reliability of the repeaters may be adversely affected.